Deep and high aspect ratio trenches are desirable in many semiconductor applications. An example of such an application is power switching applications, i.e., applications which require switching of large voltages, e.g., 25-600 V or greater. Semiconductor devices used in these applications, such as power transistors, commonly include field plate structures. Field plates are used to provide compensatory charges in the drift region of the device, thereby enabling a favorable improvement in the tradeoff between breakdown voltage rating and on-resistance. In a vertical power semiconductor device, the field plate must vertically span adjacent to the drift region of the device. This means that the field plate can span across most of the thickness of the drift region, which may represent a substantial majority of the overall thickness of the device. Accordingly, the technology used to form field plates must be capable of forming deep and high aspect ratio trenches so that the field plate can extend deeply into the drift region.
One technique for forming deep and high aspect ratio trenches is reactive ion etching. Although reactive ion etching has many advantages, several drawbacks exist. One drawback is a relatively large process window with respect to trench depth. That is, across a single wafer, the depth of trenches formed by a common reactive ion etching step can vary significantly. Another potential drawback of reactive ion etching is the formation of so-called black-silicon, i.e., needle shaped structures in the semiconductor material.